In a superconducting cable which transmits electricity with low loss, a superconducting tape is helically wound around a flexible core to form a conductor layer, so that the cable has flexibility. When a high current capacity is required, a multilayered cable with an increased number of conductor layers is used to afford a high current transmission. Examples of the high-temperature superconducting tape include a Bi-based superconducting tape and a Y-based superconducting tape. These types of superconducting tape are widely used because they have high critical current and are readily formed into a long tape.
A practical method of manufacturing a superconducting cable conductor is now described. A cylindrical stranded-wire conductor is prepared as a central support. The stranded-wire conductor is formed of multiple flexible and high-conductive wires composed of, for example, elemental Cu, alloyed Cu, elemental Al, or alloyed Al. A superconducting tape, which is 4 mm in width and 0.2 mm in thickness, for example, is wound helically around the central support leaving no space to form a first layer. Another superconducting tape is then wound helically around the outer surface of the first layer leaving no space similarly to the first layer to form a second layer. Similarly, a third layer is further wound around the second layer. Thus, multiple layers are wound over the central support to form a multilayered conductor. Then, an insulator (insulating layer) is provided on the outer surface of the superconducting cable conductor. The insulating layer is made of multiple layers of craft paper, semi-synthetic paper or synthetic paper, with the thickness of the layers corresponding to voltage. Accordingly, the insulation breakdown is avoided even when a high voltage is applied to the conductor.
In order to transmit a current through the superconducting cable conductor, it is necessary to strip off an insulating layer to expose the superconducting layer so that the exposed layer is connected with a Cu electrode. Patent Document 1 discloses a method of electrically connecting a Cu electrode (terminal member) with superconducting layers. In this method, the outer surface of each superconducting layer is covered with solder so that the superconducting layers are connected with a Cu electrode. This method achieves a small and uniform connection resistance between each superconducting layer and the Cu electrode, thus enabling even current flow in each layer, and enabling low loss and stable current transmission.
Generally, high tensile stress is applied to the electrode and the superconducting cable conductor due to a thermal shrinkage of the superconducting cable and/or a tension during installation of the cable. The above-mentioned method involving connection of superconducting layers with solder may apply large force to the superconducting tape due to such tensile stress, which leads to mechanical damage of the superconducting tape. Additionally, when a ground fault or a short-circuiting occurs in an electric power system including a superconducting cable, a fault current that is ten times to several dozen times larger in magnitude than a normal current flows instantaneously inside the power system. In such a case, the fault current exceeding a current capacity of superconducting layers may lead to burning out the superconducting layers. In order to avoid this, a central support of superconducting cable conductor is made of Cu or Al stranded wires so that the fault current flows through the central support.
The terminals of a superconducting cable conductor should be designed taking into account of such faults during operations. Patent Document 2 discloses a terminal of a superconducting cable conductor as shown in FIGS. 7 and 8. In the terminal of the superconducting cable conductor, an insulating sheath 13 is removed to expose superconducting layers 12 (a first layer 12a and a second layer 12b) over a central support 11. Moreover, the insulating sheath 13 and the superconducting layers 12 of certain lengths are removed to further expose the central support 11. A metal sleeve 30 is composed of two adjacent portions 31 and 32. A first portion 31 of the metal sleeve 30 fits around the exposed portion of the central support 11, and a second portion 32 of the metal sleeve 30 is soldered around the exposed portion of the superconducting layers 12. This structure ensures high mechanical tensile strength and provides a terminal where the first portion 32 of the metal sleeve 30 is electrically connected to the central support 11 to allow the fault current to flow.